The research is concerned with investigating the storage and usage of liquid Hydrogen and Oxygen over a long duration. For this purpose a mission was defined where these two propellants are used to transport a six man crew to Mars and back. The mission duration is a total of 972 days in length with a stopover time at Mars of 454 days. A baseline spacecraft is designed. The two driving philosophies behind the design are reliability and reusability. This baseline spacecraft design was used a a basis for analysing the extreme thermal environment and its impact on the propellant storage temperatures. Also it allowed the calculation of mass and propellant budgets. It was found that the Hydrogen fuel undergoes a change of phase when the vehicle is orbiting Mars. Hence a escape manoeuvre trajectory simulation was performed which analysed the escape trajectory, acceleration and duration, and assessed the impact on the initial Earth launch propellant budget. I addition, a number of trade-offs were performed in order to increase the efficiency of the propulsion system from its nominal design in which the Hydrogen gas is allowed to expand directly from the storage tanks through the engine. The optimum arrangement that was found was to bleed the gas into a small high pressure tank and allow the fuel to be heated by waste heat from the onboard nuclear reactor. The results indicated that not only does this provide a performance increase over the nominal system but also the amount of propellant required for this bum is smaller than the storable options considered in the literature. Hence this analysis demonstrates that Hydrogen and Oxygen can be stored and used over long periods, and that they can still provide a better propellant performance than storable options, even with the increased mass penalty associated with using them on a mission such a this one.